It is well known that ceramic, glass-ceramic and glass substrates can be decorated by transfer printing from an intaglio plate or silk screen utilizing an elastomeric silicone pad or roll as a transfer member. The decorating inks utilized in these processes are organic solutions filled with an inorganic powder. The organic solutions used for decorating at high rates (3-120 decorations/minute) are primarily fast drying lacquers and enamels. These materials have a significant drawback in that the moderately fast evaporation rates of the solvents necessitate frequent viscosity adjustments in order to maintain acceptable print quality. This problem is aggravated by high levels of inorganic fillers (pigments and fluxes) which necessitate a corresponding increase in ink solvents to maintain a printable viscosity. Alleviation of this problem by reducing solvent evaporation rates necessitates a corresponding decrease in printing rate, because a specific amount of drying is required on the elastomeric surface to achieve an ink state suitable for transfer.
Normally the print dries by evaporation and solvent penetration into the silicone. After the print dries sufficiently to form a tacky film, the print is brought into intimate mechanical contact with a substrate for which the print has greater affinity. The print consequently transfers completely to the substrate, whereupon it is further dried so that a second color may be applied from a second elastomeric pad or roll to create a multi-colored decoration. In the above processes, the ware must be contacted separately by successive heads to form a multi-colored decoration.
In the process described in U.S. Pat. No. 3,255,695, an embossed type surface was utilized to deposit successive colors of film forming inks onto a collector surface. The inks are described as having a first cohesive strength level, referred to as viscosity, and a first residual surface energy level. The collector is described as having a second residual surface energy less than the ink film. Residual surface energy differences between type face and collector are not a factor in the transfer mechanism described therein, the ink being divided (split) between the two surfaces and not transferred as an integral film. The collector elastomer roll may be utilized such that the article to be printed receives a multi-colored decoration in a single contact therewith. A time lag is provided between successive impressions to increase the cohesive strength of the previous impressions, thus preventing comingling of colors. In order to minimize this time lag and thereby achieve higher printing rates the collector surface would normally be heated. The cohesive strength of the ink could be increased by drying; but oxidation, polymerization, or, in the case of powders, melting could be employed. The cohesive strength of the ink continues to increase until it exceeds the adhesive strength between the ink and collector surface. The present invention differs from this prior art process both in the nature of the inks employed, and in the method of applying the inks onto the collector roll.
In the above disclosed process the ability to apply successive colors onto the collector surface is affected by the fact that the initial cohesive strength of the applied inks is less than its adhesive strength to either the collector surface or the previously applied ink layer thereon. Thus, the colors consequently split between the type and collector surfaces upon printing onto the collector. Previously applied colors do not contact any surfaces, except low cohesive strength ink, until such time that the composite colors are transferred to the surface to be printed.
The process of U.S. Pat. No. 3,255,695, represents an improvement over prior transfer printing processes in that it utilizes a heated pad to maintain fast printing rates, and a collector to transfer a multi-colored print in a single step. The process, however, is not without its disadvantages and limitations. Foremost, the process has all the inherent print quality limitations of typographic (embossed type) printing. The resolution, definition, and toning (shading) capabilities of typographic printing are inferior to other printing processes, especially lithographic and gravure processes. These limitations are very significant in terms of the kind and the quality of decorations which can be printed. Simple replacement of the embossed type application of ink onto the collector by other techniques is not possible since, the ink would transfer, upon contact, to all surfaces with higher residual surface energies. These include screens, gravure plates and offset pads made from conventional materials. Only by converting the collector itself to a typographic roll could a planar ink application surface be utilized.
A limitation not readily apparent in the process 3,255,695 patent is the inability to control print thickness. In glass decorating such control is often very important; normally, thermal expansion between the fluxes utilized in the design colors and the surface or substrate to be printed upon should be matched as closely as practicable. This is not always possible. For example glass ceramics sold by Corning Glass Works under the registered trademark PYROCERAM.RTM., have a sufficiently low expansion such that fluxes having all the desired expansion properties are not readily available; consequently, it is important that decorations be applied with very controlled thicknesses. The fired decorations must be thin enough not to spall during thermal cycling, but thick enough to exhibit satisfactory color intensities.
Gravure processes, not available for use in the 3,688,695 process, have an advantage when decorating such substrates because of the ability to precisely control print thicknesses by accurately controlling the depth of the engraved or etched plate. The discontinuous nature of most gravure prints, due to dot toning, is also beneficial in permitting a greater tolerance for a thermal expansion mismatch.
Decorating glass also necessitates printing with ink compositions highly filled with inorganic compounds which consequently require high levels of organic solvents to maintain acceptable print viscosities. Such solvents create toxicological and environmental problems. If color content is reduced slower process speeds result, while higher solvent content creates viscosity instability, both problems are only partially overcome by the use of a heated collector surface.
The present invention maintains a fast printing rate by utilizing a solventless ink, sometimes hereinafter referred to as a hot melt or thermoplastic ink, which is applied to a transferring elastomer from a heated intaglio surface. The ink is allowed to cool on the elastomer until a tacky cohesive film, suitable for transfer, is formed. The cooling time required on the elastomeric surface usually ranges from 0.1 to 1 second.
Although the use of thermoplastic inks in conjunction with an elastomeric transfer member is disclosed in another U.S. Pat. No. 3,688,695, such thermoplastic inks are applied to the transfer member through a heated screen, and one of two methods is then employed for transferring the image from the elastomer to a substrate. According to the first of these, the substrate is sized with an adhesive which provides a high surface affinity for the thermoplastic image; the image is then transferred to the substrate from the transfer member which has been heated to a critical tack temperature just below the melt point of the ink. According to the second method, the transferring elastomer is heated above the melt point of the thermoplastic color and pressed into intimate mechanical contact with the substrate to release the image. In U.S. Pat. application Ser. No. 74,910 filed Sept. 13, 1979, now U.S. Pat. No. 4,292,104, assigned to Corning Glass Works, the assignee herein, a decal printing device was disclosed utilizing separate printing stations incorporating offset gravure and flexographic techniques in conjunction with thermoplastic inks. That system, however, did not employ a collector.
The present invention utilizes a heated intaglio surface for applying thermoplastic ink onto a first offset (transfer) surface whereupon the ink cools to form a tacky pressure-sensitive film which is subsequently transferred by intimate mechanical contact onto a second offset (collector) surface. Such a process based on multiple offset surfaces, rather than a single offset surface as in U.S. Pat. No. 3,255,695, is compatible with gravure, as well as screen and lithographic printing. The inks herein employed are permanently pressure-sensitive, such that, when cooled upon the first offset surface they can be transferred between subsequent surfaces for which they exhibit progressively increasing degrees of affinity merely by causing intimate mechanical contact. The use of such an ink allows immediate transfer of the image from the collector to the substrate, and does not require heating of either the collector or substrate. The ink also allows for immediate overprinting of colors as no time delay is required on the collector surface to effect a cohesive strength increase. Further, the use of such an ink does not require the use of a substrate sized with an organic adhesive. The elastomeric transfer members of the present invention are continuously maintained at a temperature which is less than the temperature of the ink as applied to the transfer member from a heated etched or engraved plate or roll.
Tack is incorporated into the inks for use in the present invention through specific formulations so that heating of the releasing surface is neither necessary nor desirable. The inks utilized herein exhibit the properties of pressure-sensitive thermoplastic adhesives and are unique from other thermoplastics proposed for transfer decorating, in that the inks are transferred to the substrate in a solid or semi-solid state. For a more detailed description of specific ink formulations which may be utilized, reference is directed to U.S. Pat application Ser. Nos. 74,907 and 74,909 filed Sept. 13, 1979, now U.S. Pat. Nos. 4,280,939 and 4,261,749, and assigned to Corning Glass Works the assignee herein.
Evaluation has revealed that the inks proposed for use in the present invention exhibit many advantages over solution inks such as: viscosity stability, insensitivity to printing rate changes, insensitivity to ambient conditions, toleration of wider variable ranges on virtually all of the process variables, no deterioration of print quality due to silicone saturation or solvent evaporation, toleration of irregular ware feed, faster start-ups (no breaking-in of silicone), no solvent fumes, and simultaneous printability of both fine and bold areas.
In a copending U.S. Pat. application Ser. No. 74,910 filed Sept. 13, 1979, now U.S. Pat. No. 4,292,104, assigned to Corning Glass Works, the assignee herein, a multistage offset printing apparatus was disclosed. The present invention differs from the above identified application in that all the colors are collected on a common elastomer surface prior to offset to a substrate, thereby improving registration and significantly simplifying the construction of the apparatus.